CDMA digital cellular systems are currently in widespread use throughout North America providing telecommunications to mobile users. In order to meet the demand for transmission capacity within an available frequency band allocation, CDMA digital cellular systems divide a geographic area to be covered into a plurality of cell areas. Within each cell is positioned a base station with which a plurality of mobile stations within the cell communicate.
In general, it is desired to have as few base stations as possible, since base stations are expensive, and require extensive effort in obtaining planning permission, and in some areas, suitable base station sites may not be available. In order to have as few base stations as possible, each base station ideally has as large a capacity as possible in order to service as large a number of mobile stations as possible. The key parameters that determine the capacity of a CDMA digital cellular system are: processing gain, ratio of energy per bit to noise power, voice activity factor, frequency reuse efficiency and the number of sectors in the cell-site antenna system.
One method of achieving an increase in capacity is to replace a wide beam width antenna with an antenna array that allows the formation of a number of narrower beam widths that cover the area of the original beam. Referring to FIG. 1, a conventional CDMA communication cell 100 is shown comprising 3 adjacent hexagonal sectors, alpha 102, beta 104 and gamma 106. Each cell comprises an antenna tower platform 120 located at the intersection of the 3 sectors. The antenna tower platform 120 has 3 sides forming an equal-lateral triangle. Each sector has 3 antennas (only antennas in sector alpha 102 shown) a first antenna 114, a second antenna 116 and third antenna 112 mounted on a side of the antenna tower platform 120. Each sector also has 3 beams (only beams in sector alpha 102 shown) a first beam 108, a second beam 110 and a third beam 112. The 3 beams 108, 110, 112 are adjacent with some overlap. The 3 sectors alpha 102, beta 104 and gamma 106 are identical in structure with respect to antennas and beams. The signal for a particular user can then be sent and received only over the beam or beams that are useful for that user. If the pilot channel on each beam is unique (i.e. has a different PN (pseudo-random noise) offset) within each sector then the increase in capacity is limited due to interference between reused pilot channels in different cells.
An improvement is to use multiple narrow beams for the traffic channels and transmit the overhead channels (pilot, sync, and paging channels) over the whole sector so that the pilot channel is common to all the narrow beams used by the traffic channels in that sector. This leads to substantial gains in capacity. For example, a change from a system with a single beam per sector to a system with 3 beams per sector with a common pilot channel yields a 200 to 300% increase in capacity. It is therefore desirable that the pilot channel be broadcast over the area covered by the original wide beam. A possible arrangement is to use multiple beams per sector for the traffic channels and transmit the overhead channels over a separate wide beam antenna covering the whole sector. However, this requires the expense of extra hardware as well as the calibration and adjustment needed to match the phase of the pilot channel with the phase of the traffic channels over time and temperature.
Another possible solution is to use adaptive antenna array techniques to transmit and receive multiple narrow beams for the traffic channels and transmit the overhead channels over the whole sector on the same antenna array. However, this requires complex calibration equipment and algorithms.
Yet another solution is to use an antenna array that transmits and receives multiple sectors over fixed narrow beams for the traffic channels and transmit the pilot channel on the same fixed narrow beams. However, the problem with this approach is that the strength of the pilot channel signal at any point in the sector is determined by the vector sum of all of the pilot channel signals from each beam. Since the pilot channel signals from each beam are coherent, areas where the vector sum of the pilot channel signals is null or severely degraded will occur. This can result in dropped calls when a mobile station enters one of these areas.
There is thus an advantage to provide an antenna array that uses fixed narrow beams for transmitting and receiving the traffic channels on multiple beams and can broadcast the common pilot channel over all of the sector using the same antenna array. Furthermore, it would be advantageous to provide an antenna system that did not require complex calibration and adjustment to maintain performance over time and temperature.